In the current electronics field of semiconductor devices, the charge degree of freedom of electrons is used while electrons have the spin degree of freedom in addition to charge.
In recent years, spintronics using the spin degree of freedom have been drawing attention as the bearer of information technology in the next generation.
In spintronics, charge of electrons and the spin degree of freedom are used simultaneously so as to aim at gaining functions and properties which did not exist in the prior art.
As an initial device in spintronics, GMR (giant magneto-resistive) elements can be cited, and these use a phenomenon of change when affected by the difference in the direction of magnetization of a free layer and in the direction of magnetization of a pinned layer depending on the spin of electrons which are a carrier of a sense current that flows through the GMR element, that is to say, whether the spin is an up spin or a down spin.
In recent years, spin RAM's where the direction of magnetization in a free layer is controlled by adjusting the spins of electrons which are a carrier of a current when a current flows directly through a GMR element or a TMR (tunnel magnet-resistive) element in an MRAM (magneto-resistive random access memory) having GMR elements and TMR elements as memory cells instead of controlling the direction of magnetization in a free layer by adjusting the magnetic field generated when a current flows through a wire layer as in the prior art (see for example Patent Document 1 or Patent Document 2).
In addition, quantum computers can be cited as another mode of spintronics, and in quantum computers, quantum bits (Qubits) are provided using spins of atoms, ions or molecules (see for example Patent Document 3 of Non-patent Document 1).
Furthermore, information is conveyed by means of an electron current in current information processing apparatuses, and a current flow accompanies Joule heat.
When Joule heat is generated, there is a problem with high power consumption, because it increases as the integration of information processing units increases, and therefore, transmission of information using a spin current instead of an electron current has been investigated.
This uses the properties that an electron current of conductive electrons in a solid is a chronologically irreversible process, whereas a spin current is a reversible process with almost no dissipation of energy, and thus does not cause the power consumption to increase.
That is to say, the movement of conduction electrons is reversed when the time is reversed in the minus direction, while a spin current has a momentum of spins themselves and an angular momentum of spins, though it is generated through the movement of conduction electrons, and therefore, the momentum and the angular momentum of spins are both reversed and set off in the case where the time is reversed in the minus direction, and thus, there is no reversal as a whole, and it becomes a reversible process.
In spintronics, the concept of spin relaxation becomes very important.
In spin RAM's, for example, the relaxation time of the magnetic moment in a free layer, that is to say, the spin relaxation time of individual electrons included in the free layer determines the rate of write-in, and therefore, it is desirable for spin relaxation to be small, in order for write-in to be easy, while it is desirable for the spin relaxation to be large, in order for the rate of write-in to be high.
In addition, in quantum computers, spin relaxation determines the time for holding information, and thus, spin relaxation is important.
That is to say, it is assumed that in functioning quantum computers, the time for operation is shorter than the decoherence time of the system, that is to say, the spin relaxation time.
Thus, spin relaxation means damping of movement of spin or magnetic moment.
That is to say, the movement of spins or the magnetic moment is precession having the direction of the magnetic field as a rotational axis, and can be represented by the Landau-Lifshitz-Gilbert (LLG) equation shown below, where an attenuating term is added to the basic equation for a magnetic moment.dVM/dt=−YVM×Heff+(α/Ma)VM×(dVM/dt)
Here, Ma is the saturation magnetization, Heff is an effective magnetic field and α is Gilbert's relaxation constant.
In addition, “VM” and “VH” are used as vector symbols for the convenience of preparation of the specification.
The second term on the right side of this LLG equation is the damping, and represents the dissipation in the angular momentum and the energy of the spins or the magnetic moment, and the generation of a spin current, and the dissipation of the spin current makes it so that the spin or the magnetic moment is aligned in the direction of the external magnetic field VH after a predetermined relaxation time, and this phenomenon of generation of a spin current is known as spin pumping.
In addition, the spin-Hall effect is known to be a phenomenon relating to the effects of spins, and when a current flows through a sample, a pure spin current is generated without an accompanying current of charge in the direction perpendicular to the direction of the current, and thus, spin polarization is induced at the ends of the sample in the direction of the spin current (see for example Non-Patent Document 2).
In addition, the present inventor discovered that a current flows in the direction perpendicular to the direction of the pure spin current when a pure spin current is injected into a sample, and there is a difference in potential between the ends of the sample when the inverse spin-Hall effect is used, and thus, it becomes possible to detect whether or not there is a flow of a pure spin current by detecting the difference in potential (see for example Non-Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Publication 2002-305337    Patent Document 2: Japanese Unexamined Patent Publication 2007-059879    Patent Document 3: Japanese Unexamined Patent Publication 2004-102330Non-Patent Document 1:    http://www.s-graphics.co.jp/nanoelectronics/news/hpmolcom/2.htm    Non-Patent Document 2: Science, Vol. 301, pp. 1348, 2003    Non-Patent Document 3: Applied Physics Letters, Vol. 88, p. 182509, 2006